1. Field
Exemplary embodiments of the present invention relate to a memory system and a method for operating the same.
2. Description of the Related Art
A memory cell of a memory includes a transistor serving as a switch and a capacitor that stores charges, i.e., data. “High” (logic “1”) and “low” (logic “0”) of data are classified according to whether there are charges in the capacitor of the memory cell, that is, whether a potential between both terminals of the capacitor is high or low.
Since data retention represents that charges are accumulated in the capacitor, there is no power consumption in principal. However, data may be lost since initial charges stored in the capacitor are lost by a leakage current due to a PN junction and the like of a metal oxide semiconductor (MOS) transistor. To substantially prevent such a loss, it is necessary to read data of the memory cell before the data is lost and to recharge normal charges according to the read information. Only when such an operation is periodically repeated, data storage is substantially maintained. Such a recharge process of the cell charge is called a refresh operation.
The refresh operation is performed whenever a refresh command is applied from a memory controller to a memory, wherein the memory controller applies the refresh command to the memory at a set time interval taking into consideration a data retention time of the memory. For example, when all memory cells of the memory may be refreshed when the data retention time of the memory is 64 ms and the refresh command is applied 8,000 times, the memory controller applies the refresh command to the memory 8,000 times for 64 ms.
With an increase in the degree of integration of a memory, an interval among a plurality of word lines included in the memory is reduced, resulting in an increase in a coupling effect between adjacent word lines. Therefore, when a specific word line is excessively activated several times or is frequently refreshed in the memory during a refresh operation, data of a memory cell MC coupled to a word line adjacent to the specific word line may be lost. Such a phenomenon is called word line disturbance.